Mortality rates at older ages have been falling throughout the twentieth century. By 2050 at least 20 percent of the population of the United States is expected to be older than 64. The consequences of mortality decline for older age health are still disputed. One view holds that rising longevity may increase both chronic disease and disability rates. Another view holds that the onset of both chronic disease and disability rates may be postponed. Alternatively, even though declines in mortality may increase the prevalence of chronic disease rates, the rate of progression of chronic disease and therefore of disability may fall. Although the short-run consequences of mortality declines for older age health may differ from the long-run consequences, the evidence suggests that in the long-run population aging has been accompanied by improvements in elderly health. Several factors could account both for long-term improvements in elderly health and for increases in longevity at older ages. This project will use data on both recent and past populations to investigate the plasticity of aging to assess explanations for long-run trends in disease, disability, and death. Among the explanations considered will be reduced infectious disease rates, reduced occupational risk and improvements in socioeconomic status, and improved nutritional intake. The project will focus on the role of these factors in both older age and young adult health proxies to determine the timing of when infectious disease, occupational risk, socioeconomic status, and nutritional intake affect health declines. The project will examine the role of these factors not just in disease and mortality rates, but also in cognitive functioning at older ages. The project will also examine the social construction of disability by investigating changes in the relationship between disability and labor force participation. In addition, it will estimate the economic gains to improvements in chronic disease, disability, and death rates, apportioning the economic gains to those due to improvements in reduced infectious disease rates and to changes in socioeconomic status. The findings have implications for theories of aging; for forecasting future health and mortality trends; for assessing policies aimed at reducing the fiscal deficits in Social Security Old Age Insurance and Medicare; and for assessing the economic gains to investments in biotechnology and innovations in medical care.